Arthropod plant pests cause extensive and severe damage to major agricultural commodities, both in the field and in the greenhouse environment. In addition to feeding damage, many of these insects also transmit viral diseases, and insects such as whiteflies and aphids deposit "honey dew" on leaves, thus providing a favorable environment for the production of sooty mold which reduces photosynthetic activity and crop quality.
Infestation by the new B strain of the sweetpotato whitefly has proven particularly devastating to growers from Florida to California and as far north as New York and Ohio. The insect has a wide host range which includes over 500 species of plants. Two dissimilar species, the greenhouse whitefly and sweetpotato whitefly, alone have caused economically significant damage to poinsettia, hibiscus, tomato, crossandra and other plants in a greenhouse environment. The greenhouse whitefly, native to North America, is now world wide in distribution and is resistant to most synthetic pesticides. The sweetpotato whitefly is not limited to the greenhouse environment and is particularly difficult to control on row crops because it develops on the lower leaf surface which is difficult to adequately cover with pesticides. It also has the ability to change host plant and to acquire resistance to conventional chemical pesticides. The recent rapid spread of strain B of this whitefly has caused significant economic losses to growers of cotton, melons, squash, sugar beets, lettuce, carrots, tomatoes, peanuts, alfalfa and ornamental plants. In addition, it is a vector for more than 70 diseases including 25 viruses, and, following serious whitefly infestations, several agricultural regions have been subjected to the viral diseases which cause pepper necrosis and yellowing of lettuce.
Whiteflies are generally tropical in distribution, however the sweetpotato whitefly is now believed to have spread in the United States with impunity because of a high level of insecticide resistance and insignificant natural enemies. There have been some efforts to establish populations of parasitoids which apparently reduce or suppress the insect in its native habitat.
Chemical control of whiteflies has proven difficult for several reasons. The insect has a complex life cycle where the egg and pupal stages are generally resistant to chemicals. The entire life cycle is very short (approximately one month), resulting in a rapid increase in population. A severe infestation often occurs before a grower recognizes the problem, making eradication even more difficult. The infestations are rarely localized since the adult readily flies and the immature stages are distributed on bedding and ornamental plants. It can also develop a resistance to chemical insecticides fairly quickly, requiring control methods utilizing an alternating schedule of chemicals.
In choosing an effective pesticide, the mode of action is an important factor. The whitefly uses a piercing and sucking system to extract food from the phloem of the infested plant, and its stylets can penetrate through a dry film of pesticide on plant tissue without serious consequence from the pesticide. Therefore, either a systemic pesticide which penetrates the leaf surface or is absorbed by the roots and can be ingested by the insect or one which penetrates or acts directly on the insect are the limited approaches.
Long chain fatty acids (particularly C.sub.12) and fatty acid soaps have been reported as effective in the control of insects (Kabara, ACS Symposium Series, No. 325, 1987). In addition, various species of Nicotiana plants have been shown to have resistance to infestation by green peach aphids (Thurston et al., Ent. Exp. & Appl., 1962 and Burk et al., J. of Econ. Ent., 1969), two-spotted spider mites (Patterson et al., J. of Econ. Ent., 1974), tobacco hornworm (Jones et al., Entomol. Exp. Appl., 1985) and greenhouse whitefly (Neal et al., Tob. Int., 1987). Moreover, Cutler et al. (in Natural Resistance of Plants to Pests, American Chemical Society, 1986) studied leaf surface chemicals of N. tabacum and suggested that plant resistance to tobacco budworm is associated with the presence of a mixture of sucrose esters having an acetate group at the C.sub.6 position of the glucose moiety and a series of C.sub.3 to C.sub.8 alkanoyl groups, in particular 2-methylbutyrate and 3-methylvalerate. Johnson et al. (Tobacco Science, 1982) also studied tobacco leaf surface chemistry to determine the cause of plant resistance to aphids and found that high levels of sucrose esters or duvatriene-ols appeared to be responsible. The need therefore exists for the identification and development of the particular specific sucrose esters present in Nicotiana leaves which have the capability of controlling soft-bodied arthropod insect pests.